Sea otters have made a dramatic recovery — but now sharks are putting them in danger

Once thought to be hunted to extinction for the fur trade, the southern sea otter has had a slow road to recovery under more than a century of legal protections.

Now, the population is facing another threat: a growing number of attacks by white sharks.

State Fish and Wildlife said Tuesday that despite a recent dip in numbers, the average population count of sea otters has remained above 3,090 for the third year in a row — an important benchmark that triggers consideration by scientists to remove otters from the Endangered Species List.

Delisting sea otters wouldn’t strip away their only protections: The thick-pelted mammal also falls under the Marine Mammal Protection Act that makes it illegal for people to harass, feed, hunt, capture, collect or kill any marine mammal.

Mike Harris, a sea otter biologist with the California Department of Fish and Wildlife, examines an otter killed by great a white shark in July 2016. Joe Johnston jjohnston@thetribunenews.com

What delisting would mean is that scientists believe the species is no longer threatened with extinction.

“That would be a milestone where we say, ‘OK, the otters are on track.’ But we still have a long way to go,” said Lilian Carswell, the southern sea otter recovery coordinator for the U.S. Fish and Wildlife Service.

To decide if delisting is appropriate, she said they have to consider the threats to otter populations, including shark bite mortality, lack of range expansion and changes in prey.

This year’s average count of sea otters was 3,128. That’s 58 fewer than counted in the 2017 survey, according to data released by state Fish and Wildlife and the U.S. Geological Survey.

While the small sea otter population at San Nicolas Island continues to increase, the mainland population — stretching from Point Año Nuevo in San Mateo County to Gaviota State Beach in Santa Barbara County — decreased by 2.2 percent.

The mainland population was largest between Seaside and Cayucos.

Threat from sharks

The single greatest cause of death to southern sea otters by far is fatal shark bites — even though scientists have no evidence that sharks actually eat otters, according to both Carswell and Mike Harris, Morro Bay-based senior environmental scientist with the California Department Fish and Wildlife.

Instead of preying on the otters, sharks are taking investigative bites, scientists say, meaning the predators “meant to bite a pinniped (like a seal) and instead get this dry mouthful of fur,” Carswell said. The shark continues on its way, and the compromised otter is left with a deadly wound, sometimes with a shark tooth embedded in its side as well.

“We probably would have hit that 3,090 threshold years ago if not for the loss to the population of white shark bites,” Harris said.

And the rate of deadly attacks seems to be increasing.

While otters killed by shark bites have been found for as long as scientists have been studying them, bites are now estimated to be responsible for about 50 percent of all beached otters.

Every year since the mid-2000s, about 450 sea otters have been found stranded, sick, dead or injured, with nearly half having been bitten by a shark, Harris said. Shark bites that don’t immediately kill an otter can cause death later due to a secondary bacterial infection from the wound.

The sea otter population in Morro Bay is healthy even as the Central Coast has seen more otter deaths from shark bites. Joe Johnston jjohnston@thetribunenews.com

Searching for answers

So why is this happening?

“We don’t know. But we think that there are more sharks and we think that the distribution of sharks has changed because the pinniped distribution has changed,” Carswell said.

Adding to the theory, she said, it may be that a higher proportion of sharks are reaching adulthood thanks to restrictions on gill net fishing in areas where they grow up. Those sharks may be having difficulty transitioning from eating fish in their youth to eating mammals as adults

“They don’t know what they’re doing,” Carswell said of the juveniles.

Why sharks are biting otters is one research question. How to protect otters from the increased threat is another.

“We grapple with that. It’s our job to bring species to recovery,” Carswell said. “This is a major threat. What can we do about it? In the case of shark bites, it wouldn’t be ethical or possible to prevent shark bites.”

Instead, she suggested expanding otter territory into estuaries where sharks don’t swim. That could require addressing water quality issues, for example.

It’s a constant quandary of management to return sea life to abundant, diverse ecosystems.

“Sea mammals all were decimated in the past and they’re all coming back, but not in the most convenient ways,” Carswell said.

Sea otter awareness events

Several events are planned along the Central Coast for Sea Otter Awareness Week, Sept. 23 to 29.

The Morro Bay Museum of Natural History will host a talk called Sea Otters and Estuaries: Paradise Lost with Dr. Brent Hughes, Assestant Professor of Biology Sonoma State University from 6 p.m. to 8 p.m. Thursday, Sept. 27.

Docents will staff an information table about sea otters near Morro Rock from 10 a.m. to 2 p.m. everyday this week until Saturday.

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Monica Vaughan: 805-781-7930; @MonicaLVaughan
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